it does sound like bad news but it mite be worth cutting it hard back and see what happens in the spring . yes you can replace it but youll have to revitalise the soil .

prune it now to 3/4 clear dormant buds and then see what it does when the buds break in mid spring [depending on where you are].
when I had one there was a central column through the stem that was darker than the rest of the wood. that is normal as far as I am aware and its due to the different age of 'wood' as the plant grows. We didn't really get a good crop of grapes until it had been in place for about 4yrs.
You don't say where-abouts the plant is growing, open ground/glasshouse [though Portugal I suspect it is in open ground].
have you fed the plant? often that makes it produce leaves rather than flowers and then grapes.

an ornamentel grape vine I bought from swallow aquatics after spotting it and wanted it . after reading the description including its growing size which said 90 feet I deffinatly wanted it . it didn't have a price and only had a latin name . I asked the price of the ornimentle grape vine that grows 90 feet as im dyslexic pointing in its general direction . the woman running it looked at me old fashioned and sarcasticly said , as she bent over to look at the tag "90 feet" as if I was stupid and just felt like making up a size that suited me perhaps said ow 90 feet . after eating a large slice of humble pie sold it to me for aprox £12 . I cant imagine where she thought I got the figure from . just a mystery .anyway I digress and my spelling and grammer isn't very good if you didn't notice lol . after planting it showed that infact it easily did the 90 feet and some . one of these 90 foot stems just died from the end slowly but surly back to the base . I thought id better cut it off thinking it would carry on and kill the plant . I had to do this very aggressively in mid summer . it of course bled but came back with a vengeance anyway . ps you should see my hand writing if you think the spelling and grammer are bad . apparently the part of your brain you use for writing is the same part you use for art . as im a pretty good artist that's my story and im sticking to it lol .

Cheer up Nosey, you're a lot better at it than you used to be.

yes but I weighed 8lb then and cried and drank milk lol .